The present invention relates to an active matrix liquid crystal display device, and particularly to an active matrix liquid crystal display device which has, as switching elements, thin film transistors using semiconductor layers made of polysilicon or the like.
In recent years, liquid crystal display devices have been put to practical use, which provide high-performance and high-definition display at a high density and large capacity.
Of a number of types of liquid crystal display devices, an active matrix liquid crystal display device particularly draws the public attention. This type of display device has an array substrate in which pixel electrodes, using thin film transistors (TFTs) as switching elements, are arranged in a matrix. The liquid crystal display device is advantageous in the following respects: crosstalk between adjacent pixels is small; a high-contrast image can be displayed; transmission display is possible; and the display area can be increased easily.
The array substrate applied to the active matrix liquid crystal display device includes a plurality of scanning lines and a plurality of signal lines extending in directions crossing each other on an insulating substrate. The array substrate also includes TFTs arranged near the intersections between the scanning lines and the signal lines, and pixel electrodes arranged in a plurality of regions, i.e., pixel regions, defined by the scanning lines and the signal lines.
The active matrix liquid crystal display device has a black matrix (BM) to prevent light leakage between pixel regions. The black matrix is generally arranged along with color layers serving as color filters in a counter substrate facing to the array substrate via a liquid crystal layer. Therefore, it is necessary to take account of displacement of the positions of the array substrate and the counter substrate. If the displacement occurs, the ratio of apertures which allow passage of light, i.e., the aperture ratio, is reduced.
To solve this problem, proposed in recent years is a wiring BM structure in which a light-shielding organic insulating film, serving as a black matrix, is formed on wiring portions such as the scanning lines and the signal lines in the array substrate. In the wiring BM structure, the pixel electrode is located in the uppermost layer of the pixel region, and end portions of the pixel electrode overlap the wiring portions arranged in a matrix. Another wiring BM structure is also proposed, in which, instead of the organic insulating film, color layers serving as color filters are arranged on the wiring portions, not in the counter substrate, and used as a black matrix. In these wiring BM structures, since the aperture ratio is not reduced by displacement of the array substrate and the counter substrate, a high aperture ratio is obtained.
However, the wiring BM structures described above have the following drawbacks.
In the structure wherein the wiring portions and the pixel electrodes overlap the organic insulating film or the color layers interposed therebetween, the parasitic capacitance between a signal line and a pixel electrode arranged in different layers is greater than that between a signal line and a pixel electrode arranged in the same layer with a predetermined distance. Therefore, the image quality of the liquid crystal display device is liable to be influenced by the parasitic capacitance. To prevent this, it is necessary to provide a storage capacitance fixed to a potential in all the pixel regions.
In the active matrix liquid crystal display device, hundreds of thousands to a million or more of pixel electrodes are arranged in a matrix and electrically connected to TFTs. Therefore, it is very difficult to produce all pixel regions of all array substrates without defects: that is, pixel defects occur in a certain ratio. There are various reasons for pixel defects. A defect analysis has made clear that most pixel defects are impairment due to a short circuit between electrodes constituting a storage capacitance. If such impairment occurs, the pixel is fixed to a certain potential, resulting in a defect that the pixel is always lighted. Further, since a DC voltage is continuously applied across the pixel and the counter electrode, the liquid crystal composition contained in the liquid crystal layer corresponding to the pixel region is deteriorated, with the result that the reliability is lowered.
One of the methods for repairing the pixel defects is to apply a laser beam to the storage capacitance electrode where a short-circuit defect occurs, thereby electrically cutting it from the pixel electrode. In this case, the repaired pixel is improved to a half-lighted state, although it is influenced by the parasitic capacitance between the signal line and the pixel electrode.
However, in the aforementioned wiring BM structures, if a part of the wiring portion is to be cut by a laser beam, a new short-circuit defect may occur, since the wiring portion overlaps the pixel electrode. To avoid this, if a wiring portion to be cut is formed so as not to overlap the pixel electrode, light will pass through the portion, resulting in a low contrast.
The present invention was made to overcome the problems described above. Its object is to provide an active matrix liquid crystal display device in which a display defect can be repaired without lowering the contrast.
To achieve the above object, according to claim 1, there is provided an active matrix liquid crystal display device comprising:
an array substrate including: a scanning line; a signal line crossing the scanning line; a switching element located at an intersection between the scanning line and the signal line and electrically connected to the signal line; a pixel electrode electrically connected to the switching element through a first coupling line; a storage capacitance electrode electrically connected to one of the switching element and the pixel electrode through a second coupling line; and a storage capacitance signal line facing the storage capacitance electrode via an insulating layer; and
a counter substrate having a counter electrode which faces the pixel electrode with a liquid crystal composition inserted therebetween,
wherein the second coupling line includes a portion which is exposed through the storage capacitance signal line.
According to claim 7, there is provided an active matrix liquid crystal display device comprising:
an array substrate including: a scanning line; a signal line crossing the scanning line; a switching element located at an intersection between the scanning line and the signal line and electrically connected to the signal line; a pixel electrode electrically connected to the switching element through a first coupling line; a storage capacitance electrode electrically connected to the switching element through a second coupling line; and a storage capacitance signal line facing the storage capacitance electrode via an insulating layer; and
a counter substrate having a counter electrode which faces the pixel electrode with a liquid crystal composition inserted therebetween,
wherein the switching element includes a semiconductor layer, and a part of the semiconductor layer ranging from a channel region formed in the semiconductor layer to a connecting portion between the first and second coupling lines includes a portion which is exposed through the other lines.
According to claim 8, there is provided an active matrix liquid crystal display device comprising:
an array substrate including: a scanning line; a signal line crossing the scanning line; a switching element located at an intersection between the scanning line and the signal line and electrically connected to the signal line; a pixel electrode electrically connected to the switching element through a first coupling line; a storage capacitance electrode electrically connected to one of the switching element and the pixel electrode through a second coupling line; and a storage capacitance signal line facing the storage capacitance electrode via an insulating layer; and
a counter substrate having a counter electrode which faces the pixel electrode with a liquid crystal composition inserted therebetween,
wherein the storage capacitance signal line includes a first portion which overlays the storage capacitance electrode to form storage capacitance and a second portion which is exposed through the other lines.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.